Reduction of acidity of polyesters by melt reaction endcapping with dialkyloxylate or tetraalkyl urea

ABSTRACT

A method for the melt reaction of polyesters, particularly polyethylene terephthalate, with an endcapping agent which reacts with terminal carboxyl groups of the polyester. The endcapping agents are selected from the group consisting of dialkyloxylates and tetraalkylureas.

BACKGROUND OF THE INVENTION

Polyesters utilized in fiber formation are generally produced by aheated reaction of one or more dibasic acids such as terephthalic acid,or the like, with one or more polyhydroxy compounds such as ethyleneglycol, propylene glycol, 1,4-cyclohexane dimethanol, or the like, untila product of desired viscosity is obtained. The formed polyesters arecharacterized in that they contain both terminal hydroxy and carboxygroups. Terminal hydroxy groups are generally more predominant due tothe incorporation of an excess of polyol in the reactive mixture.

Polyesters are of great importance in the manufacture of tire cords, andas reinforcement for belts, hoses and many other useful articles. Inmany of these commercial applications the presence of excessive carboxylgroups in the polymer molecule is detrimental.

Previous attempts at acid group reduction in polyesters have resulted ina loss of average molecular weight in the polyester product due tosubstantial cleavage in the polyester backbone.

It is an objective of the instant invention to provide improvedpolyester materials in which the pendant carboxyl groups are eithergreatly reduced in number or are completely removed.

It is a further object of the invention to endcap free carboxyl groupson polyesters without producing water as a byproduct and whilemaintaining the molecular weight of the polyesters.

It is a further object of the instant invention to provide polyestermaterials having reduced sensitivity to water.

SUMMARY OF THE INVENTION

The instant invention relates to melt reaction of a carboxyl groupcontaining polyester with a carboxyl group reactive endcapping agentselected from the group consisting of dialkyloxylates andtetraalkylureas, in which the alkyl groups are lower alkyl radicals; toprovide a polyester having a substantially reduced number or no carboxylgroup while maintaining the approximate molecular weight of the carboxylgroup containing polyester precursor.

DETAILED DESCRIPTION OF THE INVENTION

In the practice of the present invention the polyester material is firstproduced in any state of the art commercial manner. A typical processfor production of a polyester is the heated reaction of a basicdifunctional organic acid with a polyol, preferably a diol, optionallytogether with any other desired components.

Suitable polyesters for treatment in the instant invention are preparedfrom difunctional organic acids including, but not limited to:terephthalic acid, 4,4'-dicarboxydiphenyl, 1,5-,1,4-, or 2, 6-naphthalicacid and the like. Suitable polyols are preferably diols such as, butnot limited to, ethylene glycol, propylene glycol, butylene glycol, andthe like. The preferred polyesters of the instant invention arehomopolyesters such as polyethylene terephthalate,poly-1,4-cyclohexylenedimethylene terephthalate, polyethylene-2,6-naphthalate, polyester ethers such as polyethylene hydroxybenzoate,poly-p-phenylene bis-hydroxyethoxy-benzoate, poly-p-phenylenebis-hydroxyethoxy-terephthalate; copolyesters or copolyester etherswhich comprise mainly ethylene terephthalate units or tetramethyleneterephthalate units and other copolymer components such astetramethylene or ethylene isophthalate, 1,4-cyclohexylenedimethyleneterephthalate units, or tetramethylene or ethylene p-hydroxybenzoateunits, or the like. The preferred polyester for treatment in the instantinvention is polyethylene terephthalate. Polyesters for treatment in theinstant invention have an acid value ranging from 40 to 10 equivalentsof CO₂ H per 10⁶ gm of polyester. Polyesters for treatment in theinstant invention should have an average molecular weight ranging from10,000 to about 60,000.

In the practice of the instant invention a formed polyester is meltreacted with a carboxy reactive group or endcapping agent selected fromthe following groups: dialkyloxylates and cyclic tetraalkylureas inwhich the alkyl groups are lower alkyl radicals. The use of any of theseendcapping agents permits the treated polyesters to retain theirapproximate molecular weight and viscosity as significant amounts ofwater which would promote polymer degradation are not generated duringendcapping reaction.

The dialkyloxylates which can be utilized in the instant invention arerepresented by the following structural formula: R₁ OOCCOOR₂ (I) whereinR₁ and R₂ represent the same or different C₁ -C₅ alkyl radicals. Thepreferred dialkyloxylates are dimethyloxylate and diethyloxylate.

The dialkyloxylates of the present invention are utilized to endcapcarboxy terminated polyesters by the following reaction: R₁--OOCCOOR_(2+Polyester--COOH)→Polyester--COO(R₁ or R₂)+HOOCCOO(R₁ or R₂)wherein R₁ and R₂ are as defined above. The alkyl group which esterifiesthe carboxylpolyester is, of course, replaced in the dialkyloxylate by ahydrogen atom to yield an alkyl-oxalic acid monoester.

The tetraalkyl urea compounds which are utilized as endcapping agentsare represented by formula (II): ##STR1## wherein R₆₋₉ are the same ordifferent C₁ -C₅ alkyl radicals or R₈ and R₉ taken together form aheterocyclic radical with a nitrogen atom. The preferred tetraalkyl ureacompound is bis(pentamethylene) urea or other symmetric ureas.

The tetraalkyl urea endcapping agents of the instant invention endcapcarboxy terminated polyesters by the following reaction: ##STR2##wherein R₆₋₉ are as previously defined.

In the process of the present invention the melt extrusion reaction ofthe polyester and the appropriate endcapping agent should occur in atemperature range between 270° C. and 320° C. The endcapping agent feedrate into the melt extruder should range between 1 and 50 millimoles perminute per 100 grams per minute of polyester feed. The reactionresidence time of the polyester and the endcapping agent in the meltreaction must be at least 10 seconds to provide for substantialendcapping of the carboxyl groups present on the untreated polyesterwhich is fed into the melt reactor. This residence time allows forendcapping of the acid group thereby effecting acid number reduction ofthe polyester to an acid number below 10 equivalents of CO₂ H per 10⁶ gmof polyester, preferably below 3 eq. CO₂ H/10⁶ gm of polyester.

The polyesters produced in accordance with the instant procedure havingless than 10 equivalents of CO₂ H per 10⁶ grams of polymer are accordedthe status of having substantially all of their carboxyl groupsendcapped.

The following examples are presented for the purposes of clarifying thepresent invention. However, it should be understood that they are notintended to limit the present invention in any way.

The following are specific examples for each of the above groups of theendcapping agents and their use in capping the free carboxyl groups inpolyesters. In all of the following examples the treated polyester ispolyethylene terephthalate.

In each of the following examples the polyethyleneterephthalate (PET)melt was prepared as follows. Tirecord grade PET was continuouslyprepared from terephthalic acid and ethylene glycol to give an intrinsicviscosity, [η], of 0.94 dl/gm at 25° C. in 1:1 ratio ofphenol:tetrachloroethane. The PET in chip form was dried at 110° C. forat least twelve hours in a rotary dryer under a vacuum of 0.10 mm of Hg.The recovered dry PET polymer was transferred to an Acrison No. 1015Z-Cfeeder under a nitrogen atmosphere and fed to a Werner-Pfleiider ZSK-30twin screw compounding extruder which had all zones heated to either280° C. or 300° C. At a polymer feed rate of 40 gm/min or 100 gm/min thePET polymer had a melt residence time of 85 or 35 seconds, respectively,in the extruder.

EXAMPLE 1

The compounding extruder zones were heated to 280° C. and the feed rateof the PET polymer was 87 gm/min. During the PET melt residence time ofabout 40 seconds the last part of zone one of the compounding extruderwas continuously injected with 10.4 millimoles (mM)/min (1.07 cc/min) ofthe capping agents dimethyloxalate using a BIF microfeeder No. 1180-07piston pump. The extruded polymer was cooled, chopped and analyzed todisplay an intrinsic viscosity [η]=0.73 dl/gm and 1.8 eq CO₂ H/10⁶ gm ofPET polymer.

In comparison, a control PET polymer passing through the compoundingextruder without the added dimethyloxalate displayed [η]=0.72 dl/gm and23.8 eq.CO₂ H/10⁶ gm of polymer.

EXAMPLE 2

In accordance with the procedure of Example 1, PET polymer was fed intoa compounding extruder heated to 280° at a feed rate of 23 gm/min havingan extruder residence time of about 2.5 minutes. During the melt time,bis(pentamethylene) urea was injected into the extruder at a feed rateof 2.00 cc/min (10.4 millimoles/min). The extruded polymer was cooled,chopped and analyzed to show an [η]=0.54 dl/gm and 4.2 eq CO₂ H/10⁶ gmof polymer. A noncapped extruded PET polymer displayed an [η]=0.80 dl/gmand 41.2 eq CO₂ H/10⁶ gm of polymer.

We claim:
 1. A method for reducing the acidity of polyesters whichcomprises melt reacting a carboxyl group containing polyester having anacid value ranging from 40 to 10 equivalents of CO₂ H per 10⁶ gm ofpolyester with an endcapping agent selected from the group consisting ofdialkyloxylates and tetraalkylureas in which the alkyl groups are loweralkyl radicals having from 1 to 5 carbons; in an amount ranging from 1to 50 millimoles per minute for each 100 grams per minute of polyesterfeed at a temperature in the range of 270° C. to 320° C., therebyeffecting endcapping of the carboxyl groups of the polyester to an acidcontent below 10 milliequivalents of CO₂ H per 10⁶ gm of polyester. 2.The method according to claim 1 wherein the melt reaction of thepolyester with the endcapping agent occurs for a period of time of atleast 10 seconds.
 3. The method of claim 1 wherein the carboxyl groupcontaining polyester is polyethylene terephthalate.
 4. The method ofclaim 1 wherein the endcapping agent is dimethyloxylate.
 5. The methodof claim 1 wherein the endcapping agent is bis(pentamethylene) urea. 6.A polyethylene terephthalate resin composition comprised of the reactionproduct of a carboxyl group containing polyethylene terephthalate resinhaving an acid content greater than 10 equivalents of CO₂ H per 10⁶grams of polymer and a carboxyl group reactive endcapping agent selectedfrom the group consisting of dialkyloxylates and tetraalkylureas inwhich the alkyl groups are lower alkyl radicals having 1 to 5 carbonatoms: to produce a polyethylene terephthalate resin having an acidnumber below 10 equivalents of CO₂ H per 10⁶ grams of polymer.
 7. Thepolyethylene terephthalate resin composition of claim 6 wherein theendcapping agent is dimethyloxylate.
 8. The polyethylene terephthalateresin composition of claim 6 wherein the endcapping agent isbis(pentamethylene) urea.